1. Field of the Invention
The present invention relates to wireless communication systems and signal processing apparatus employed in wireless communication systems. The term ‘wireless communication systems’ includes cellular communication systems, personal communication systems (PCS), wireless local loop systems, and all other like systems.
2. Background of the Prior Art and Related Information
Wireless communication systems employing transmission between base stations and remote users are a key component of the modern communications infrastructure. These communication systems are being placed under increasing performance demands which are taxing the capability of available equipment, especially wireless base station equipment. These increasing performance demands are due to both the increasing numbers of users within a given wireless region, as well as the bandwidth requirements allocated to wireless system service providers. The increasing number of wireless users is of course readily apparent and this trend is unlikely to slow due to the convenience of wireless services. The second consideration is largely due to the increased types of functionality provided by wireless systems, such as wireless Internet access and other forms of wireless data transfer over such systems. These considerations have resulted in a need for more communication channels per transmit carrier and more transmit carriers operating from each transmitting location of a wireless service network.
There are several methods for creating multiple communication channels on a single carrier. These methods include code division multiple access (CDMA), time division multiple access (TDMA), as well as others. In each of these methods, several data channels enter a signal generator that combines the input data channels using one or more of the methods mentioned above to produce a complex signal output. FIG. 1 shows such a single carrier communication system having multiple user channels. The system has a signal generator 1 receiving plural user data channels D1 . . . Dn and producing a complex pair of signal outputs labeled V1 and V2. These signal outputs are then filtered 2, digital to analog converted (DAC) 5, RF up conversion modulated 6, amplified 7, and transmitted 8. The filter 2 used in this process performs two important functions. First, the filter limits the bandwidth of the signal exiting the signal generator. The bandwidth of transmission is regulated by government requirements. Second, the filter impulse response determines the trajectory of the signal waveform. The signal trajectory is an important part of the signal modulation format and is used to reduce interference in signal reception.
A key drawback of the signal generation and transmission method given in FIG. 1 is power amplifier efficiency. In such communication systems, the efficiency of the amplifier 7 must be reduced to achieve good linearity. Without good linearity the spectral bandwidth of the transmit signal would be increased during amplification. Excessive increase in signal bandwidth may cause the transmit signal to exceed the government regulated limits.
To improve efficiency in transmission the transmitter shown in FIG. 2 was developed. This transmitter includes the signal generator 1 and filter 2 of FIG. 1 but then uses a method known as envelope elimination and restoration to produce the transmit signal exiting the power amplifier 7. The first step in envelope elimination and restoration is to convert the transmit signal from rectangular coordinates to polar coordinates. The rectangular to polar coordinate converter 11 produces two outputs based on the two rectangular coordinate inputs provided by the filter 2. The first output represents the signal envelope or signal gain. The second output represents the signal normalized to the signal envelope or the signal phase. Each time sample of the phase signal output is a unity amplitude phasor at a phase angle determined by the instantaneous phase of the modulated signal output from filter 2. The phase component is then digital to analog converted (DAC) 5, RF up conversion modulated 6, and power amplified 7. An alternative to this method would have the phase signal output from the rectangular to polar converter 11 represent the derivative with respect to time (frequency) of the instantaneous phase of the modulated signal output from the filter 2. This alternate signal would then be digital to analog converted (DAC) 5, and converted to a RF signal by application to a voltage controlled oscillator 6, and power amplified 7. Since the signal amplified by the power amplifier contains no amplitude information the linearity of the power amplifier is not a consideration. The power amplifier can therefore be designed for high efficiency. To restore the envelope to the transmitted signal the gain component is digital to analog converted (DAC) 12, and provided to a power supply modulator 13 that provides source power to the power amplifier. The signal envelope therefore controls the gain of the amplifier. The rectangular to polar coordinate converter 11 eliminates the signal envelope and the power supply modulator provides envelope restoration.
To achieve accurate signal generation using envelope elimination and restoration one key limitation is generally placed on the transmit signal. This limitation is that transmit signal amplitude does not approach near or cross zero signal amplitude. If the signal amplitude approaches near or crosses zero amplitude the gain of the transmit amplifier must be set very low or to zero by the supply modulator. Low or zero gain settings mean that the dynamic range of the supply modulator measured in decibels must be very large. Large dynamic range modulators that maintain accurate signal level are difficult to construct. In addition, when approaching very near or crossing zero amplitude the phase component of the signal will be changing very rapidly or instantaneously. This causes the bandwidth of the phase component to be very broad. High bandwidths increase the required signal processing sample rates and the cost of components found in the phase signal path. Fortunately, when generating single carriers for transmit several modulation formats exist, and others can be easily determined, where the signal amplitude does not approach near or cross zero. For such systems, transmit signal generation is often performed using envelope elimination and restoration.
As mentioned above however, increasing user demand is requiring greater numbers of communication channels per transmit carrier. To increase the number of communication channels per transmit carrier transmit signal modulation formats without restriction on signal amplitude variation have been selected. Spread spectrum modulation formats such as code division multiple access (CDMA) and orthogonal coded time division multiple access are examples of transmission formats without limitation on amplitude variation.
Also mentioned above, increasing user demand is requiring the transmission of several transmit carriers from one location. When combining transmit carriers, the signal amplitude of the combined carriers will vary without limit due to the phase combination of multiple carriers at different frequencies. The selection of modulation format cannot eliminate small or zero crossing amplitudes of the multiple carriers. FIG. 3 shows current methods of combining multiple carriers with each carrier composed of one or more communication channels. FIG. 3 shows that after filtering 2 each carrier is offset in frequency 3, combined 4, analog to digital converted (DAC) 5 modulated 6, power amplified 7 and transmitted 8. Envelope elimination and restoration techniques are generally not used in such prior art communication systems due to the combined signal zero crossings.
Accordingly, a need presently exists for a solution to the problems associated with signals crossing or approaching zero in wireless communication systems.